RFID tags utilize the RFID (Radio Frequency Identification (Identification Technique based on Radio Communication)), and are thin, light and small electronic devices, each of which has a semiconductor chip for storing data, such as an identification number, and an antenna for transmitting and receiving radio waves.
Such RFID tags are expected to be widely utilized in various service environments in various fields, such as physical distribution management, and are desired to be mass-produced to reduce the production costs thereof to be spread. Antennas for RFID tags are required to have a low electrical resistance in order to increase the data transmittable/receivable range (communication range) thereof to reduce data loss during transmit/receive. Moreover, RFID tags are used in various fields, such as physical distribution management (e.g., management of tracking of shipping containers, traceability and positional information, and management of closing by laundry, such as laundry tags), so that they are often repeatedly bent in service environment. Therefore, even if RFID tags are repeatedly bent, it is required to prevent them from being unserviceable as RFID tags by the deterioration of characteristics of antennas, such as breaking and increasing of electrical resistance due to metal fatigue of antennas, so that they are required to have good flexibility.
As methods for forming antenna circuits (conductive circuits) for RFID tags, there are a method for utilizing a copper coil or wire as an antenna, a method for transferring a metal foil, such a copper or aluminum foil, to a substrate, a method for printing an etching resistant ink as an antenna circuit pattern on a metal foil, which is laminated on a substrate such as a plastic film, to mask it to etch the metal foil, and so forth.
However, since these methods are not suit for mass production due to the limitation of their productivity, it is difficult to further reduce the production costs. In the method for transferring a metal foil to a substrate and in the method for etching a metal foil among the above-described methods, the metal foil is produced by rolling or the like, and the percentage of a metal in the metal foil is a high percentage which is approximately 100%. For that reason, there is a problem in that an RFID tag having an antenna circuit formed by a metal foil has bad flexibility although it has good electrical characteristics. In addition, although a metal foil having a thickness of about 10 to 50 μm is generally used for forming an antenna circuit for an RFID tag, if the metal foil is too thick, the characteristics of the metal foil approach those of a metal plate for deteriorating the adhesion thereof to a substrate, so that there is some possibility that the metal foil may be stripped from the substrate when the RFID tag is bent. Moreover, since the percentage of the metal in the metal foil is high, when RFID tag is bent, stress concentrates on the bent surface thereof, so that cracks are easy to be generated on the bent surface thereof. As a result, the electrical characteristics thereof are deteriorated, and the breaking thereof is caused, so that it does not function as an antenna for an RFID tag. On the other hand, if the percentage of a metal is decreased by using a conductive film of the metal component and a resin component in place of the metal foil in order to improve the flexibility of the RFID tag, it is possible to generally improve the flexibility by stress relaxation, but the amount of the metal component is decreased for deteriorating the electrical characteristics thereof, so that it does not have sufficient characteristics as those of an antenna for an RFID tag.
As a method for producing an antenna for an RFID tag wherein a conductive circuit is formed on a substrate so as to have good adhesion thereto without the use of any metal foils, there is proposed a method for applying a water based conductive ink containing 40% or less by weight of silver particles on the surface of a film substrate by the flexographic printing to dry the ink to form a conductive film having a thickness of 0.1 to 0.5 μm on the surface of the film substrate to produce an antenna for an IC tag which is one kind of RFID tag (see, e.g., Japanese Patent Laid-Open No. 2010-268073).
In the method disclosed in Japanese Patent Laid-Open No. 2010-268073, it is possible to mass-produce antennas for IC tags, which have a low electrical resistance, to reduce the production costs thereof. However, the conductive ink containing the small amount of silver particles is used for forming the thin conductive film having the thickness of 0.1 to 0.5 μm, and the percentage of silver in the conductive film is a high percentage which is approximately 100%, so that there is a problem in that the flexibility of the tag is bad similar to that in the method for transferring a metal foil to a substrate and in the method for etching a metal foil.
In an RFID tag chip integrated with an antenna, there is a problem in that the communication range thereof is short, so that it is desired to increase the communication range thereof. For that reason, it is desired to increase the communication range thereof by mounting a booster antenna on an RFID tag chip integrated with an antenna, the booster antenna being formed so as to be electromagnetically coupled to the antenna of the RFID tag chip.
However, when the booster antenna is mounted on the RFID tag chip, it is required to mount thereon the booster antenna, which has excellent electrical characteristics and flexibility and which can be inexpensively mass-produced, similar to the antenna integrated with the RFID tag chip.